Method and an apparatus for drying wood



L. MALMQUIST METHOD AND AN APPARATUS FOR DRYING WOOD Filed Nov. 13, 1967 July 22, 1969 3 Sheets-Sheet l LARS MALMQUIST WW ATTYS.

1969 L. MALMQUIST 3,456,356

METHOD AND AN APPARATUS FOR DRYING woon Filed Nov. 13, 1967 3 Sheets-Sheet :3

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FIBRE SATURATIQN MOISTURE CONTE N1 TEMPERATURE OF woon-c Ul o 16% 2'02: ab 2: 4'0 z 5'0 7. 6'0 2 7'07, 602 9'07:

MOISTURE CONTENT INVENTORL LARS M ALMQUI 5T ATTYS.

United States Patent 3,456,356 METHOD AND AN APggATUS FOR DRYING W Lars Malmquist, Huddinge, Sweden, assignor to Aktiebolaget Svenska Flaktfabriken, Stockholm, Sweden Filed Nov. 13, 1967, Ser. No. 682,067 Claims priority, application Sweden, Nov. 14, 1966, 15,517/ 66 Int. Cl. F261) 3/04, /12

US. Cl. 34-1 10 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a method for drying wood from a moisture ratio exceeding the fibre saturation moisture ratio to a moisture ratio below the fibre saturation moisture ratio.

It is well known that wood products must be dried with great care for obtaining an acceptable quality. By practical tests in chamber dryers, therefore, drying schedules, i.e., instructions concerning the dry and wet bulb temperatures V and V respectively of the drying medium as a function of the moisture prevailing in the wood during the drying have been developed, which instructions give different recommendations for different wood species and wood thicknesses. Such recommendations have been worked out particularly in wood research institutes in England and U.S.A. They will be dealt with in detail later on in connection with the description of the accompanying figures.

The complications occurring during the drying of wood are due primarily to the changes in shape and dimension which are caused in the wood cells upon their drying and which express themselves in undesirable deformations of the wood, in crack formation or in remaining drying tensions. The method according to the invention is based on the knowledge that the stress conditions resulting from said changes in the shape and dimension of the cells during the drying are of different nature when the Water leaves the cell cavities (lumina) and the cell walls (gel system). The water in the cell cavities, the free water, is retained in the cell by capillary forces, while the water in the cell walls, the bound water, is retained by stronger molecular forces in the gel system. During the drying process, first the free water is removed and thereby the cell cavities are emptied. First thereafter the cell walls are dried out. The moisture ratio at which the cell cavitie are dried out, but the cell walls still are Water saturated, is called the fibre saturation moisture ratio and lies at about 30% (and decreases slightly at increasing temperature).

During the drying of wood with a moisture ratio exceeding the fibre saturation moisture ratio, capillary suction forces come into existence at the water transport to the wood surface where evaporation takes place. The suction effect caused hereby tends to reduce the volume of the cell cavities which gives rise to a certain so-called cell reduction which in extreme cases can result in a so-called 7 Patented July 22, 1969 venting these complications, the drying conditions must be so chosen that the cells are sufficiently elastic to withstand without plastic deformation the capillary forces and to remain intact until the fibre saturation moisture ratio is obtained at which the effect of the capillary force ceases. This can be achieved by a sufficiently low temperature of the wood in this phase of the drying and by simultaneously maintaining the capillary suction effect moderate, by a sufiiciently low drying speed.

During the drying of wood with a moisture ratio below the fibre saturation moisture ratio, the hygroscopically bound Water in the cell walls is removed, which causes the cell walls to shrink. This shrinkage is both anisotropic, i.e., of different size in different directions, and nonhomogenous, due to different shrinkage properties in different parts of the wood (for example knots, compression wood, spiral growth, spring wood and summer wood) and due to the moisture ratio gradient in the wood subsequent to the drying. Consequently, shrinkage stresses brought about during the drying below the fibre saturation moisture ratio can be balanced to a substantial degree only by plastic deformation of the wood or by outer deformations of the wood or crack formation. As outer deformations (crookedness, twisting, etc.) and crack formation are not desired, the drying conditions must be so chosen that plastic deformation can take place.

This is possible when the temperature of the wood is sufiiciently high. There exists, namely, a limit temperature below which the wood is substantially elastic and above which it is substantially plastic, i.e., where stress conditions effected can be balanced by an internal flow in the wood. The dependency of the limit temperature on the moisture ratio of the wood-the elasto-plastic limit curve-has been empirically determined by experiments (H. Kiibler: Plastische Formung und Spannungsbeseitigung bei Holzern, unter besonderer Beriicksichtigung der Holztrocknung. Holz als Rohund Werkstoff, vol. 14, 1956, page 442). Whereby the limit temperature, for example close to the fibre saturation moisture ratio, was found to lie at about 35 C., and at 8% moisture ratio at about C. No experiments were made at moisture ratios above the fibre saturation moisture ratio, but the limit temperature there remains constant at about 35 C., because at the fibre saturation moisture ratio successively are filled with water. As the water in the cell cavities cannot influence the elasto-plastic properties of the cell walls, the properties, thus, are the same as at the fibre saturation moisture ratio, and the limit temperature, therefore, remains unchanged at about 35 C.

For preventing damages of the wood by drying, the drying conditions, in view of the different nature of the stress conditions caused during the drying above and below the fibre saturation moisture ratio, must be so chosen that the wood temperature at moisture ratios exceeding the fibre saturation moisture ratio must be below the elasto-plastic limit curve, but at moisture ratios below the fibre saturation moisture ratio the wood temperature must be above said curve. The invention is characterized in that the drying is carried out in two steps, in such a manner that the wood in the first step is dried to a moisture ratio close to the fibre saturation moisture ratio and during this time is held at a temperature which at any moment is lower than the elasto-plastic limit temperature at the corresponding moisture ratio, and that in the second step the wood is dried from the moisture ratio close to the fibre saturation moisture ratio to the desired final moisture ratio, whereat at the beginning of the second step, before the moisture ratio has had time to change substantially, the temperature of the wood is caused to increase to a value exceeding the elasto-plastic limit temperature at the corresponding moisture ratio, and therea after, until the wood is dried substantially completely, is held at any moment at temperatures exceeding the elasto-plastic limit temperature at the corresponding moisture ratio.

When wood products are to be dried at a low temperature in circulating air, a relatively great amount of ventilation air must be used, because the capacity of the air to accumulate water vapour decreases strongly with decreasing temperature. In order to prevent the heat consumption per kilogram of water removed by drying from becoming too high, it is necessary to recover heat from the outgoing ventilation air to the incoming ventilation air. The heat recovery can best be effected, from a tech nical point of view in so-called tunnel dryers in which the wood is advanced by steps from the feed end of the dryer to its discharge end, and in which the condition of the drying medium surrounding the wood is constant all the time in any point of the dryer.

Drying at low temperature, therefore, is preferably carried out in a tunnel dryer. In a modern industry with a high output of wood products this dryer type offers also the advantage of a greater possibility for continuous operation than in chamber dryers, in which a wood charge lies still and the drying medium surrounding the wood varies during the time. Moreover, the tunnel dryer, particularly such with longitudinal circulation of the drying medium, is much cheaper in relation to its capacity than the chamber dryer, with respect to both initial and operation costs. A further development of the tunnel dryer is therefore of great technical interest.

According to one embodiment of the method the invention is applied to the drying of wood in a drying tunnel which is divided into two zones. The invention is characterized in that the wood is advanced through the tunnel in such a manner, that the Wood after having reached a moisture ratio close to the fibre saturation moisture ratio is fed over from the first zone to the second zone. By this method an adjustment of the wood temperature can be obtained while maintaining the tunnel drying principle, which adjustment effectively contributes to a reduction of damages caused in the wood by drying. The wood temperature increase intended to take place in the second zone of the drying tunnel can be ef fected by supplying the wood in the second zone with electric high-frequency energy or by subjecting the wood to a circulating drying medium of a higher wet bulb temperature than in the first zone.

The invention is particularly adapted for the drying of wood products in the form of sawn wood or in another form, for example veneer, from undried state to a moisture ratio substantially below the fibre saturation moisture ratio (842%), but it also is adapted, especially in the alternative with high-frequency heating in the second zone, for the drying of new-sawn wood to about 20% moisture ratio (export wood) in such cases when a short drying time is of importance for a desired high production capacity.

An apparatus for carrying out the method according to the invention for the drying of wood comprises a drying tunnel adapted to feed therethrough by steps wood which is stacked on conveyors in packages separated from each other. The apparatus is characterized in that the drying tunnel is divided into two zones whereof the second one is adapted to impart to the wood a higher temperature than in the first zone.

An embodiment of the apparatus is characterized in that the second zone is adapted to supply to the wood electric high-frequency energy. This can be effected either in a hollow space which is fed with electric energy of ultrahigh frequencies, or by-passing the wood between condenser plates which are connected to a generator for high-frequency alternating voltage. Another embodiment is characterized in that the zones are separated by a gate which can be opened, and that each of the zones is provided with separate means for blowing through the zone in question a gaseous drying medium and with means for adjusting the condition of the drying medium in the zone in question.

The properties of the drying method are further dealt with in connection with the description of the accompanying drawings wherein:

FIG. 1 shows a horizontal section of an embodiment of the apparatus for drying according to the invention,

FIG. 2- shows a vertical section through another embodiment,

FIGS. 3 and 4 show the temperature curve of the wood during the drying when it is dried according to traditional recommendations, and

FIG. 5 shows the corresponding temperature curve obtained by the application of the invention, compared with the curve obtained with a conventional dryer with longitudinal circulation.

In FIG. 1 a plurality of wood packages 1 is placed outside of the receiving door 2 of a wood dryer 3 which is of the tunnel type with longitudinal circulation. The dryer 3 is divided into two zones 4 and 5 which are separated by a gate in the form of a roller shutter 6. In the zones 4 and 5 six and four drying wood packages 7 and 8 respectively located in the drying tunnels. The completely dried wood packages leave the dryer intermittently through a discharge door 9 in the discharge end of the dryer 3. Simultaneously with the discharge of a completely dried package, the receiving door 2 and the roller shutter 6 are opened, and all the remaining packages are advanced by one step whereafter a new package is fed in at the receiving door.

The two zones 4 and 5 are equipped in substantially like manner. Each of them comprises two return air ducts 10 and 11 respectively arranged on both sides of the drying chamber proper and provided with circulation fans 12 and 13 respectively and heating elements 14 and 15 respectively for the heating of the circulating drying air. The heating elements are fed with hot water. In their return ducts are monted motor valves 16 and 17 respectively which are controlled by regulators 18 and 19 respectively. To the return air ducts 10 are connected ducts 20 for taking in ventilation air, and ducts 22 with motoroperated dampers 24 for the discharge of ventilation air. The ducts 11 have openings 21 for letting in ventilation air and openings 23 with motor-operated dampers for letting out ventilation air. Said inlet ducts 20 and outlet ducts 22 are connected to heat exchangers for the recovery of evaporation heat from outgoing wet air to ingoing fresh air.

The motors for the dampers 24 and 25 are controlled by regulators 18 and 19 respectively. The regulators are fed with measuring values for dry bulb temperature from transmitters 26 and 27 respectively or for wet bulb temperature from transmitters 28 and 29 respectively, which are mounted in the return air ducts. The control equipment 16, 17, 18, 19, 24, 25, 26, 27, 28, 29 maintains the temperature and moisture of the circulation air constant, before the air circulates around the wood.

By this arrangement the circulating dry air in the second zone can be given a higher temperature and higher moisture and, thus, a higher wet bulb temperature of the air in said zone than in the first zone. Since in convective drying a wet material to be dried assumes a temperature which is substantially equal to the wet bulb temperature of the drying air, the wood in the second zone is given a higher temperature than it had in the first zone. The length of the zones and the drying intensity must be so balanced that the wood is fed over from the first zone to the second zone when its moisture ratio is equal to the fibre saturation moisture ratio.

FIG. 2 shows an embodiment of the invention with dielectric heating of the wood in the second zone. A plurality of wood packages wait outside of the door 41 of a tunnel dryer 42 through which the wood is to be fed in steps. The wood is stacked on carriages adapted to be rolled through the dryer 42 on a conveyor 43. Five wood packages 44 are in a first zone of the drying tunnel, and two wood packages 45 in a second zone thereof. Outside of the discharge end of the dryer 42 are located two completely dried woodpackages 46.

A fan 47 mounted above the drying tunnel can blow air through the drying tunnel in its longitudinal direction. The air is heated in a first step in the hot water heating element 48 which comprises in its return line for hot water a control valve 49 actuated by a regulator 50 which is connected to a temperature transmitter 51 located in the air flow after the element. The air is thereafter wetted to saturation by spraying in water through nozzles 52. The air is then heated in a second step in a hot water heating element 53, in the hot water return line of which is mounted a control valve 54 actuated by a regulator 55 which is connecttd to a temperature transmitter 56 located in the air flow after the element 53. By this apparatus equipment the desired wet bulb temperature of the air can be automatically adjusted. The wood then assumes the same wet bulb temperature throughout the first zone.

In the second zone the wood packages pass between two condenser plates 57 and 58 which are connected to and fed with high-frequency alternating voltage from a high-frequency generator 59 transforms the mains frequency to a high-frequency alternate voltage. The wood in the wood packages 45 constitutes the dielectric between the condenser plates 57 and 58, and within the wood dielectric losses are caused which are transformed to heat and heat the wood. The air from the fan 47 passes also along the wood packages in the second zone and thereby cools the wood, the temperature of which is determined by the equilibrium between heat supplied dielectrically and heat removed convectively. The woodtemperature in the second zone, therefore. can be influenced either by changing the condenser voltage, and thereby the dielectric losses, or by changing the air flow through the fan 47, for example by a throttle valve before the fan.

For being able to understand how the wood tempera ture changes during the drying and to compare the wood temperature at drying after traditional recommendations with the wood temperature in a drying process according to the invention, one may set out from thte known fact that as long as free water is found in the wood cells, i.e., at moisture ratios above the fibre saturation moisture ratio, the wood temperature is close to equal to the wet bulb temperature V of the drying medium, and that at moisture ratios below the fibre saturation moisture ratio the wood temperature gradually increases to the dry bulb temperature V of the drying medium at decreasing moisture ratio, said increase taking place at the beginning with an accelerating rate and in a later phase with a retarding rate, when the wood moisture ratio approaches the equilibrium moisture ratio of the Wood which corresponds to the dry and wet bulb temperatures. With a knowledge of the drying schedule, i.e., the temperature V and V as a function of the moitsure ratio of the wood, it is possible in this way to estimate the wood temperature as a function of the moisture ratio.

FIGS. 3 and 4 show wood temperatures obtained in this way (with a scale on the vertical axes of the diagram) as a function of the moisture ratio of the wood (with a scale on the horizontal axes of the diagram) for different traditional drying schedules. The wood temperature thus obtained is accurate at moisture ratios above the fibre saturation moisture ratio and approximate at moisture ratios below the fibre saturation moisture ratios, but sufficiently accurate for the comparison intended. The fibre saturation moisture ratio which decreases somewhat with increasing wood temperature, has also been indicated in the figures as a steeply inclined curve. The elasto-plastic limit curve is drafted as a dashed curve.

FIG. 3 shows the wood temperature obtained when some drying schedules designated by A, B, E and H were applied. Said schedules had been tested at Forest Products Research Laboratory, England, and are representative for a great number of recommendations for difierent wood species and wood thicknesses. The curve of the wood temperature can be characterized as a substantially vertical parallel displacement of the elasto-plastic limit curve. This implies that the wood temperature according to these drying schedules substantially at all moisture ratios lies either entirely above or entirely below the elasto-plastic limit curve. Therefore, it does not meet the conditions of the' invention that the wood temperature at moisture ratios above the fibre saturation moisture ratio shall be lower than the elasto-plastic limit temperature, and at moisture ratios below the fibre saturation moisture ratio shall be higher than said limit temperature.

FIG. 4 shows in the same manner the wood temperature obtained when some representative drying schedules designated by T2-C1, T6D4, T10-F5, T12-C5 and T14-B5 were applied which had been tested at Forest Products Laboratory, Malison, Wis, U.S.A. Neither these recommendations meet the condition according to the invention, but result in a wood temperature which for all moisture ratios lies above the elasto-plastic limit curve and (respectively, for the schedule T2-C1 shows a wood temperature which at moisture ratios above the fibre saturation moisture ratio lies above the limit curve and at moisture ratios below the fibre saturation moisture ratio lies substantially below the limit curce, i.e., reverse to the condition according to the invention.

FIG. 5 shows in a corresponding way the wood temperature obtained by the drying in a conventional dryer with longitudinal circulation (curve 2) in which the wet bulb temperature of the drying air is maintained at about 32 C. and the dry temperature in the discharge end at about 46 C. which are normal values for the drying of export wood, and in an apparatus according to the invention (curve 1) in which the wet bulb temperature in the first zone, i.e., at the drying down to about 30% moisture ratio, is maintained at 32 C., and in the second zone, i.e., at the drying from about 30% moisture ratio and downwards, is maintained at about 50 C., and the dry temperature in the discharge end is maintained at about 64 C. In the calculation, the two dryers compared have been assumed to have the same capacity, the same dryer length (length of wood flown through) and to give the same drying time at drying from about moisture ratio to about 10% moisture ratio. The wood temperature according to curve 2, i.e., in the conventional dryer, lies all the time below the elasto-plastic limit curve, while the temperature according to curve 1, i.e., in the apparatus according to the invention, down to about 30% moisture ratio lies below the limit curve and thereafter increases rapidly to a value above the limit curve. The wood temperature thereafter lies above the limit curve until the wood is substantially completely dried, i.e., the wood temperature is characterized by the condition according to the invention. When the wood is being transferred from the first zone to the second zone, in which the drying air has a higher wet bulb temperature, the wood temperature at the beginning lies below the dew point in this zone, whereby water Will be precipitated from the drying air into the wood, so that its moisture ratio increases somewhat during the heating. This is favorable from a wood quality point of view, because thereby the transition between the zones is moderated.

I claim:

1. A method of drying wood in two successive stages comprising the steps of subjecting the wood to a gaseous drying medium during a time sufficient for drying the wood to a moisture ratio close to the fibre saturation moisture ratio, controlling during this drying stage the temperature of the drying medium so as to impart to the wood a temperature, which at any moment is lower than the elastoplastic limit temperature for the corresponding moisture ratio, continuing the drying of the wood having a moisture ratio close to the fibre saturation moisture ratio during a second stage for a time sufficient for drying the wood to the desired final moisture ratio and imparting to the Wood during this second drying stage a temperature, which at any moment exceeds the elasto-plastic limit temperature for the corresponding moisture ratio. I

2. A method according to claim 1, wherein the wood is dried in a continuous path of travel divided into two zones, including the, step of controlling the rate of advance of the wood along the path so that the wood is fed over from the first zone to the second zone when the wood has a moisture ratio close to the fibre satuation moisture ratio. l

3. A method according to claim 2 wherein the wood in the second stage is subjected to a second drying medium including the step of circulating said media separately in each stage transversely with respect to the longitudinal direction of the path of travel.

4. A method according to claim 2 wherein the wood is subjected to a drying medium being circulated in the longitudinal direction of the path of travel including the step of passing said medium through both the first and the second zone in series.

5. A method according to claim 2 including the step of supplying in the second zone high-frequency energy to the wood in a condenser field, in which the wood is the dielectric.

6. A method of controlling a tunnel dryer to dry wood therein, said tunnel dryer having two drying zones and means to advance wood successively through said zones, means to circulate drying medium through at least said first zone to dry the wood therein and means to elfect drying the wood in the second zone, comprising the steps of first controlling the condition of said drying medium relative to the rate of advance of the wood through said first zone to dry the wood in the first zone to a moisture ratio close to the fibre saturation moisture ratio while maintaining the temperature of the wood in said first zone below its elasto-plastic limit temperature for the corresponding moisture ratio, and second controlling the drying means in the second zone to dry the wood to the 8 desired final moisture ratio while maintaining the tempe'ratufe of the wood above the elas'to-plastic temperature. 7. A method according to claim 6' wherein said first controlling step comprises controlling both the wet and dry bulb temperatures of said drying medium in correlation with the rate of advance of the Wood through'said first zone. 8. A method according to claim 6 wherein the drying means in-the second zone comprises separate means to supply a drying medium in said second zone, said second controlling step comprising controllingboth the wet and dry bulb temperatures of said drying medium in the's'ec- 0nd zone in correlation with the rate of advance of the wood through said second zone. 9. A method according to claim 6 wherein said drying means in-the second zone includes condenser plates on opposite sides of said conveyor and means'to supply high frequency alternating voltage to said plates to constitute the wood therebetween a dielectric, said second controlling step including controlling the voltage and frequency applied to said plates thereby control the heating of the Wood therebetween by the dielectric losses therein. 10. A method according to claim 9 including the step of drawing cool air over the wood in said second zone and controlling the flow of air relative to said voltage and frequency applied to said plates to thereby control the temperature of the wood therein.

References Cited UNITED STATES PATENTS 2,397,615 4/1946 Mittelmann 34-1 XR 2,567,983 9/1951 Wood 34-17 XR 2,706,344 4/ 1955 Vaughan 343l 3,131,034 4/ 1964 Marsh 34-30 2,231,457 2/ 1941 Stephen 34l KENNETH W. SPRAGUE, Primary Examiner US. Cl. X.R. 34 1s, 68 

